SUMMARY Impaired axonal development and axonal degeneration underlie many debilitating neurodegenerative disorders including hereditary spastic paraplegia (HSP), amyotrophic lateral sclerosis, and periphery neuropathy. HSPs are a heterogeneous group of more than 70 genetic disorders characterized by progressive lower limb spasticity due to a length-dependent degeneration of corticospinal motor neuron axons. SPG11 and SPG15, the most common autosomal recessive forms of HSP, are caused by mutations in KIAA1840 and ZFYVE26 that encode spatacsin and spastizin protein, respectively. Knockdown of spatacsin and spastizin in zebrafish led to abnormal axonal outgrowth and locomotor impairment. However, how altered spatacsin and spastizin activities lead to axonal defects and why specific axons degenerate in HSP patients are largely unclear. The goal of this proposed study is to establish human neuronal models of SPG11 and SPG15 to delineate the mechanisms underlying axonal defects in HSP. Based on strong preliminary data, this study?s hypothesis is that impaired autophagy and lysosomal function caused by perturbed spatacsin and spastizin levels result in axonal defects selectively in cortical PNs, which is mediated by mitochondrial dysfunction. This hypothesis will be tested by pursuing the following two aims: 1) to determine the effect of perturbed spatacsin and spastizin on axonal development and degeneration; 2) to determine the role of impaired mitochondrial dynamics in axonal defects in HSP. By comparing axonal defects, spatacsin and spastizin levels, mitophagy, and mitochondrial dynamics in cortical projection neurons, cortical interneurons, and spinal motor neurons derived from control and SPG11/SPG15 iPSCs, this study will delineate the cell type- specific defects in HSP and the underlying mechanisms. The cause-effect relationship between loss of spatacsin and spastizin function and axonal phenotypes will be confirmed by knocking out spatacsin and spastizin in wild-type neurons and by correcting spatacsin and spastizin mutations in SPG11 and SPG15 iPSCs, respectively. Moreover, rescue experiments will be performed to identify potential approaches for rescuing axonal pathology, such as correcting mutations of spatacsin and spastizin, treatment with fission/fusion targeting agents, and genetically regulating mitochondrial dynamics. Together, this study will provide valuable insights into understanding the role of mitochondrial dysfunction in HSP pathology and developing new therapeutics for rescuing axonal degeneration in HSP.